I believe a very good dlc for Skyrim would one that has more of an epic journey theme to it. A whole new area where the main quest is starting at point A and getting to point B. And then point B may or may not lead you to an unexpected point C. There would be no need for fast traveling because there would be no back tracking.
In this dlc you will be traveling a long distance with various obstacles in the way. You might run into a small village that will send you on a side quest, and it might be a go fetch quest, but it is near the village to keep from needing to fast travel. Unlike a lot of quest in Skyrim where you have to go across the map just to find someones lost sword for them or kill a master vampire. Also these villages would contain inns for a place to stop and rest before you continue on your journey.
Also, to keep it lengthy, have a giant mountain blocking your path and the only way through is a vast dungeon that has means for exploring. And mountains that you have to climb up and down to get past. Also, maybe have a major conflict going on that needs to be settled so that you continue traveling to point B.
Hopefully you get the point, I may not be explaining it as well as I'm thinking it.
Putting around 20 hours into something like this, I think, would be a very fun and unique addition to Skyrim. No more running around all over the place. Just enjoying the scenery as you progress at a steady pace.
Oh and then after completing the main quest, locations open up for fast travel if you wanna see and do some things you may have missed. I don't know, just brainstorming. A lot of things can be added to this idea to keep it interesting though I don't expect to ever see something like this in a TES game. They seem to like having run all over the place fetching and killing things for people.

